Fiber forming polyamides and other valuable polymeric products are prepared by condensing substantially chemically equivalent amounts of diamines and dicarboxylic acids. One convenient procedure is to form the diamine dicarboxylate salt of the reactants and to effect condensation by heating.
In order to prepare useful polymers in a practical way it is desirable that the reactants be as pure as possible.
It is apparent then that a method of obtaining crystalline diamine dicarboxylate would be an important contribution to the art.
The term diamine dicarboxylate as used herein refers to a salt which is the neutralization product of one mole of a diamine and one mole of a dicarboxylic acid.
A number of methods have been derived for isolating the desired salts in pure form once they have been formed.
For example, U.S. Pat. No. 2,130,947 describes crystallization from an organic solvent such as methanol. British Pat. No. 1,034,307 and Russian Pat. No. 215,937 also refer to the use of alcohol solvents. German Pat. No. 1,124,507 describes a method of obtaining the salts from solutions containing them by spray drying at elevated temperature.
These processes suffer a number of recognized deficiencies. For example, it is preferable to avoid the use of organic solvents because of ever-present fire and explosion hazzards, together with odors which are often offensive. Moreover, the salt may be contaminated with ester formed by reaction between the selected alcohol and the dicarboxylic acid. The spray drying technique avoids the problems with solvents, but is still not completely satisfactory, because the salts tend to decompose at high temperatures.